Photo-curing technology, as a novel and advanced technology for treating material surface, has been developed rapidly since the realization of industrialization in 1960s. Particularly, its features such as high efficiency, superior quality, environmental-friendly and energy-saving enable photocurable products to be applied more and more widely. Photoinitiator is a key component of photocurable materials and plays a decisive role on the photo-curing speed of the material. When traditional low-molecular photoinitiators are used, after photocurable material coatings are cured, part of the photoinitiator absorbs light energy and decomposes into free radical which will initiate polymerization and further combine with other free radicals at the end of the polymerization, either being connected to the polymer chain, or existed in the polymerization network in the form of fragments of lower molecular weight together with residual un-decomposed photoinitiators. As time goes by, they will shift to the coating surface gradually, which will influence the appearance and performance of the coating and make the coating to become yellow, and even cause toxicity effect and influence the application on foods and sanitary packing materials. In order to overcome above-mentioned disadvantages of low-molecular photoinitiator, macromolecular photoinitiator or polymerizable photoinitiator is designed. Macromolecular photoinitiator can be connected to the chain of a macromolecule or an oligomer via carbon or oxygen atoms. At present, commercial macromolecular photoinitiator comprises KIP150 oriented KIP series and KT series products of Italy Lamberti, Speedcure series products of British Lambson and benzophenone oxygen ethanoic acid ester series containing polyether polyols as the center disclosed by Chinese Patent CN1599713; CN101434543A disclosed benzophenone oxygen ethanoic acid ester with aliphatic diol as the core; CN101172951A disclosed the preparation of hydroxyl ethyoxyl benzophenone carboxylic acid ester; ZL200710090821.9 disclosed macro-molecularization of phenyl benzophenone; Chinese Patent CN1887913 disclosed the preparation of polyurethane type linked benzophenone; Polymerizable photoinitiator can incorporate polymerizable functional group such as 4-hydroxybenzophenone acrylate or metacrylic acid ester into the photoinitiator structure so as to enable the realization of marco-molecularization in the photo-curing process. This kind of products were summarized in an review named “Research Progress of Macromolecular Photo-initiator” (Shanxi Chemical Industrial Press, 2007, 27(6), 22) by Li Xiaohong.
However, macromolecular or polymerizable photoinitiators both have disadvantages of complex manufacturing process, high production cost and low product purity. For example, during the acroleic acid esterification process of 4-hydroxybenzophenone, since the initiator end and propylene end are all active groups, the esterification reaction of hydroxy is hard, and acryloyl chloride and organic amine acid binding agent must be used. Macromolecular initiators will usually have quite large molecular weights after polyether and other molecular chains are incorporated into them, and the content of effective groups is usually in an amount of only 50-60%, and compared with the corresponding low-molecular photoinitiator, the initiation activity is reduced, therefore the use dosage must be increased so as to main a proper initiation efficiency, thus the performance of the formula is reduced and the cost is increased, and customers would not like to use it, not conducive to popularization and wide application.
The photoinitiator derivatives with sulfur as bridge bond are less. U.S. Pat. No. 3,903,322 disclosed the application of 4,4′-sulfo-double benzophenone as photoinitiator in 1974, however, it can be hardly used commercially due to the high melting point of 171° C. and the low solubility; later, an improved product, i.e., 4-(4-methyl benzene sulfenyl)diphenyl ketone (BMS), had a certain application value, but the disadvantage of low solubility in acrylate monomer still existed, and the molecular weight thereof was only 304.4, so it did not have an ideal anti-mobility.
The research group of Wu Shikang in Photosensitive Chemical Research Institute Of Chinese Academy of Sciences reported a paper named “Research on Photolysis of Sulfo-phenyl Benzoate and Photo-initiation Polymerization” (Chemical Journal of Chinese Universities, 1998, 56, 979-85) where thiophenol was esterified with benzoic acid; in a Poland paper (Polymery, 2009, 54(3). 202-8), 4-mercapto benzophenone was esterified with p-methoxybenzoic acid, and was used for studying photo-initiation polymerization of metacrylic acid ester. This kind of photoinitiator did not become a commercial product due to some problems such as the difficulty of esterification reaction, stability of ester and dissolubility, and stink caused by various thioether compounds contained in the decomposition products.
U.S. Patent Application Publication No. US2008/0132601 disclosed a thiol photoinitiator, which fix the molecular in the polymerization network through the addition polymerization of thiol group and ethylene double bond under the action of free radical catalyst. This kind of thiol photoinitiator is prepared by substitution reaction of polythiol and chlorinated benzophenone. In fact, as the nature of raw material polythiol is similar to the nature of the product, the polythiol contained in the crude product almost cannot be separated, causing the first problem that products with good purity can be hardly obtained; the addition of polythiol compounds and ethylene double bond was studied early (Li, Photographic Science and Photochemistry, 1991, 9, 112), and it has been found that the addition efficiency was unideal, and the conversion rate of thiol is only 50%, which was the second problem; in view of above-mentioned problems, this patent further proposed adding a pre-polymerization step, where thiol and acrylate were pre-polymerized in the presence of catalyst of amine compounds, forming a larger molecular weight pre-polymer containing benzophenone group, which was equal to acrylate resin commonly used in photocurable compositions, and by this way the stink of thiol molecular was removed. The third problem is that, for users of the materials, the addition of the pre-polymerization step increased operation complexity and uncertainty, and the residual amount of thiol could be hardly controlled without detecting, and the odor quality and migration rate of the cured film could not be ensured.